The present invention relates, in general, to optical communication systems, and, more particularly, to an add/drop wavelength switch for wavelength division multiplex (WDM) optical communications.
Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different optical wavelengths, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, such as, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into individual channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels into output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.
Currently, filters based on fiber Bragg gratings (FBG) are among the most popular add/drop wavelength filters used in WDM networks for the add/drop operation. Another approach in the add/drop operation is the use of an array-waveguide-grating filter. In both cases, the add/drop operation is always on, which, it is believed, is not a very effective way to utilize the optical channel. Although another 2xc3x972 optical switch can be integrated with the FBG such that an add/drop operation can be controlled by a switching gate, this is fundamentally cumbersome and ineffective. Furthermore, optical switches available in the market are mostly mechanical optical switches that are not suitable in network wavelength routing because of their short lifetime (i.e., a moving motor wears out in time) and high power consumption. Although other types of optical switches are available, such as thermal optical switches, crosstalk in such switches is generally too high to permit large scale systems to be provided.
The present invention combines the characteristics of add/drop operation of a filter and the switching capability of an optical switch. The add/drop wavelength switch has at least two input ports for the incoming WDM signal and the add signal, and at least two output ports for the WDM pass-through signal and the drop signal. The wavelength switch is operated in two modes, referred to as the bridge state and add/drop state, respectively. In the bridge state, the incoming WDM signal continuously flows through the optical node without being disturbed. When controlled to do so by either the local optical node or the WDM network, the wavelength switch changes to the add/drop state in which a pre-defined optical channel is dropped from the WDM signal and the add signal is substituted into the WDM signal. The add signal can be a single channel or multiple channels. A unique feature of this add/drop wavelength switch is that the pass-through channels are not disturbed by the transition during switching between states. This assures the uninterrupted flow of WDM signals through the network. Based on this feature, an optical token ring can be realized in which multiple add/drop wavelength switches are cascaded. An array of these add/drop switches can be used to implement a wavelength crossbar that enables optical channels to be arbitrarily exchanged between multiple WDM networks.
An optical add/drop wavelength switch is controllably changed from a bridge state, in which output is identical to input, e.g. a Wavelength Division Multiplexed (WDM) input, and an add/drop state, in which a signal input to an add port is substituted for a particular wavelength subrange of the WDM input, other wavelengths of the WDM input being unchanged. In one embodiment, the wavelength subrange of the WDM signal is given a polarization different from other wavelengths of the WDM, such as by using a stacked waveplate or other optical filter or polarization discriminator. The differently-polarized wavelengths can the be spatially separated, e.g. by a birefringent element or a polarization beam splitter, preferably in a bit-controlled fashion, such as by using a liquid crystal or other polarization controller. Polarization controllers and discriminators can be used similarly to selectably align or combine the add signal with the portion of the WDM signal outside the subrange. The add/drop wavelength switch can be used, e.g. in an optical token ring network and/or to make-up an optical crossbar for exchanging any arbitrarily designatable channels, e.g. among a plurality of multi-channel optical fibers.